Golf club head manufacturers and designers are constantly looking for ways to improve golf club head performance, which includes the forgiveness and playability of the golf club head, while having an aesthetic appearance. Generally, “forgiveness” can be defined as the ability of a golf club head to compensate for mishits, i.e., hits resulting from striking the golf ball at a less than an ideal impact location on the golf club head. Similarly, “playability” can be defined generally as the ease in which a golfer having any of various skill levels can use the golf club head for producing quality golf shots.
Golf club head performance can be directly affected by the moments of inertia of the club head. A moment of inertia is the measure of a club head's resistance to twisting upon impact with a golf ball. Generally, the higher the moments of inertia of a golf club head, the less the golf club head twists at impact with a golf ball, particularly during “off-center” impacts with a golf ball. The less a golf club head twists, the greater the forgiveness of the golf club head and the greater the probability of hitting a straight golf shot. In some instances, a golf club head with high moments of inertia may also result in an increased ball speed upon impact with the golf club head, which generally translates into increased golf shot distance.
In general, the moment of inertia of a mass about a given axis is proportional to the square of the distance of the mass away from the axis. In other words, the greater is the distance of a mass away from a given axis, the greater is the moment of inertia of the mass about the given axis. To reduce ball speed-loss on off-center golf shots, golf club head designers and manufacturers have sought to increase the moment of inertia about a golf club head z-axis extending vertically through the golf club head center of gravity, i.e., Izz. By increasing the distance of the outer periphery of the golf club head from the vertical axis, e.g., the further the golf club head extends outward away from the vertical axis, the greater the moment of inertia (Izz), and the lesser the golf club head twists about the vertical axis upon impact with a golf ball and the greater the forgiveness of the golf club head.
United States Golf Association (USGA) regulations and constraints on golf club head shapes, sizes and other characteristics tend to limit the moments of inertia achievable by a golf club head. For example, the highest moment of inertia (Izz) allowable by the USGA is currently 5,900 g·cm2 (590 kg·mm2).
Because of increased demand by golfers to hit straighter and longer golf shots, golf club manufacturers recently have produced golf club heads that increasingly approach the maximum allowed moment of inertia (Izz). Although golf club heads with high moments of inertia (Izz) may provide greater left-to-right shot shape forgiveness, such benefits are contingent upon the golfer being able to adequately square up the club face prior to impacting the golf ball. For example, if the golf club head face is too open on impact with a golf ball, the ball will have a tendency to fade or slice. The harder it is to rotate the golf club head during a swing, the more difficult it is to square the golf club head prior to impact with a golf ball and the greater the tendency to hit errant golf shots. Often, the bulkiness or size of a golf club head can negatively affect the ability of a golfer to rotate the golf club head into proper impact position. In other words, because the mass of bulkier golf club heads is distributed further away from the hosel and shaft, the moment of inertia about the shaft is increased making it harder it is to rotate the golf club head about the shaft during a swing.
Conventional golf club heads approaching the maximum allowable moment of inertia (Izz), tend to be bulkier than club heads with lower moments of inertia due to the outward extend of the periphery of the golf club head. Although the bulkiness of the golf club heads may provide a higher moment of inertia (Izz) for greater forgiveness, such benefits tend to diminish as the bulkiness of the golf club head makes it harder for a golfer to square up the golf club head. In other words, the high forgiveness of the golf club head can be negated by the inability of the golfer to square the club face due to the bulkiness of the golf club head.